About 10,000 light years away, in the constellation Centaurus, is a planetary nebula called NGC 5307. A planetary nebula is the remnant of a star like our Sun, when it has reached what can be described as the end of its life. This Hubble image of NGC 5307 not only makes you wonder about the star’s past, it makes you ponder the future of our very own Sun.
The process of a star aging and reaching the end of its life is a long, slow story, punctuated with episodes of rapid change. Just like NGC 5307 did, our Sun will eventually become a red giant, casting off its outer layers of gas. Some billions of years in the future, it will itself become a white dwarf, lighting up the layers of gas it shed as a planetary nebula.
Right now, our Sun is on the main sequence. It’s fusing hydrogen into helium inside its core. As a result of that fusion, an enormous amount of energy is released, heating the Earth and keeping life going here. (It’s actually not the fusion itself that produces most of the heat; it’s the proton-proton chain.)
But a star is a balancing act between the outward pressure of the fusion, and the inward pressure of its own gravity. That balance is called hydrostatic equilibrium, and it can’t last forever.
Year by year, century by century, aeon by aeon, the Sun keeps fusing hydrogen into helium, releasing heat, and losing mass. Even though a star like our Sun can seem stable and unchanging, nothing in nature is unchanging. The Sun fuses about 600 million tons of hydrogen into helium every second, losing mass as it does so. It loses mass by converting matter into energy, as explained by Einstein’s E=mc².
That’s a significant amount. In fact, in its approximately 4.5 billion years of life so far, the Sun has lost an amount of mass similar to the mass of Jupiter.
Eventually the balancing act will be changed forever, because the Sun will lose enough mass that the inward force of its gravity will not be enough to contain the outward force of its fusion. The star will expand into a red giant.
Astronomers calculate that when our Sun becomes a red giant, in about 5 billion years, it will expand enough to engulf Mercury, Venus, and probably Earth. Leading up to that, the Sun will become roughly twice as luminous as it is now. At that point, Earth will receive about as much energy from the Sun as Venus does now. Not a good prognosis for life.
After its red giant phase, the Sun will become a sub-giant. It’ll double in size over the course of a half-billion years. Then comes another half-billion year phase where it doubles in size again, and also becomes up to two thousand times brighter. At this point, the Sun is now an enormous, bright, menacing object that has turned red and consumed the inner planets in the Solar System.
At this point, the Sun will be on the red-giant branch. It’ll have a core of helium surrounded by a layer of hydrogen. After billions of years of active life, the Sun will have only about 100 million years of active life left. But there’s a lot of activity compressed into that 100 million years.
First there’s the helium flash, where the Sun will burn 40% of its mass. It’ll do that by converting about 6% of the helium in its core into carbon. That’ll take only a few minutes, a shocking juxtaposition against the billions of years in the Sun’s life.
After losing all that mass, it’ll shrink to about 10 times its current size, and about 50 times its luminosity. At that point, the Sun is on the horizontal branch, and it will continue to burn the helium in its core for the next hundred million years, becoming a little larger and more luminous.
But now the Sun is running out of fuel. The helium in its core is being further depleted, and it’s losing more mass. Nothing can stop this from happening, and the Sun will expand again, as it did when it first entered the red giant phase. But this expansion will be a lot more rapid.
Things are speeding up for the Sun, and it’s becoming increasingly unstable. Our once-implacable Sun is entering its final stages. It’s now in the asymptotic-giant-branch phase, and will spend about 20 million years in the early part of that phase. It has a mostly inert core of oxygen and carbon, a shell where helium is fusing into more carbon, and another shell where hydrogen is fusing into helium. There’s a lot going on.
It will convulse in a series of thermal pulses and mass loss. Each of these pulses last only a hundred years or so and in each one the Sun will expand and become more luminous. Each pulse will be stronger than the one preceding it, and this period lasts about 100,000 years. Calculations show that our Sun will likely experience four of these pulses near the end of its life.
After being wracked by these pulses, the Sun will calm down. The Sun, for all intents and purposes, is dead. Or at least in a coma. The pulses have shed its outer layers, and it’s now a white dwarf. This white dwarf will only contain about 50% of the Sun’s original mass.
The Sun is dead because there’s no fusion anymore. As a white dwarf, it emits only stored energy. It’s made up of densely-packed electron-degenerate matter, and no fusion can take place.
But it’s still shining, and the energy it emits strikes the layers of gas it shed during its thermal pulses, ionizing the gas and lighting it up. Our Sun will then be a planetary nebula. And that brings us back to NGC 5307.
NGC 5307 is a glimpse forward to the end of the Sun’s life. Just like NGC 5307, our Sun will one day, billions of years from now, be only a remnant of its former glory as a life-giving ball of plasma. Despite the planetary nebula name, there will be no planets nearby. It will have destroyed them during its expansions. There will only be the gas.
But even the gas will eventually be gone. It’ll move away from the star and cool. After about 10,000 years as a planetary nebula, the former Sun will persist as a feeble white dwarf for trillions of years. After that, according to theory, the Sun will become a black dwarf. It will have cooled completely and emit no energy. This is theoretical because no black dwarfs have been observed. In fact, it takes longer for a star to evolve to this hypothetical black dwarf state than the age of the Universe itself so far.
The expelled gas from the planetary nebula still has a role to play. Throughout the chaos of the Sun’s latter stages of evolution, it produced elements heavier than hydrogen and helium through stellar nucleosynthesis. These elements, called metals in astronomy, will be sent out into space and taken up in another stellar formation process. They’ll enrich the next star to be born, and the next planets that might form around this future star.
The name planetary nebula is a misnomer from earlier days in astronomy. They’re not related to planets in any way. But some of the first observers of these stellar remnants, with the telescopes available to them at the time, saw the rounded shapes and assumed they were planets.
Now we know that’s not true. We now recognize them for what they are. Each one of these nebulae is like a snapshot summing up the billions of years it took to reach this state. And though it will never be observed by human eyes (probably,) this is the eventual fate of our Sun.
There is an enormous amount of detail in the life and eventual death of a star. When we say something like “fusing hydrogen into helium releases heat” there’s a lot more to it, and a lot more than can fit into one article.
If you want to know more about stars, I recommend “The Life and Death of Stars” (2013) by Kenneth R. Lang. Lang is a Professor of Astronomy at Tufts University, and he does an excellent job of explaining all things stellar.
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